One electrophotographic image-forming apparatus known in the art is a printer provided with a main casing, and process cartridges that are detachably mounted in the main casing. Each process cartridge comprises a drum cartridge that retains a photosensitive drum, and a developer cartridge that retains a developing roller for supplying toner to the corresponding photosensitive drum. This type of printer has been provided with a structure for bringing the developing roller in each developing cartridge to a position adjacent to the photosensitive drum of the corresponding drum cartridge in order to supply toner to the photosensitive drum during image formation, and to separate the developing roller from the photosensitive drum out of the image formation so that toner cannot be supplied to the photosensitive drum.
One example of this type of printer that has been proposed provides the main casing with a main frame formed of sheet metal, left and right fixing members for fixing the photosensitive drums in position relative to the main frame, and separating members for placing the developing rollers in contact with the corresponding photosensitive drums and for separating the developing rollers from the corresponding photosensitive drums. Each drum cartridge is provided with a photosensitive drum having a drum shaft on its right end, and a protection part in which the left end of the photosensitive drum is inserted; and provides each developer cartridge with developing-roller bearing members in which both left and right ends of the developing roller are inserted.